Baoliang Chen

Fast and slow rates of naphthalene sorption to biochars produced at different temperatures.

This study investigated the sorption kinetics of a model solute (naphthalene) with a series of biochars prepared from a pine wood at 150-700 °C (referred as PW100-PW700) to probe the effect of the degree of carbonization of a biochar. The samples were characterized by the elemental compositions, thermal gravimetric analyses, Fourier transform IR spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller-N(2) surface areas (SA), and pore size distributions. Naphthalene exhibited a fast rate of sorption to PW150 owning a high oxygen content and a small SA, due supposedly to the solute partition into a swollen well-hydrated uncarbonized organic matter of PW150. The partial removal of polar-group contents in PW250/PW350, which increased the compactness of the partition medium, decreased the diffusion of the solute into the partition phase to result in a slow sorption rate. With PW500 and PW700 displaying low oxygen contents and high SA, the solute sorption rates were fast, attributed to the near exhaustion of a partition phase in the sample and to the fast solute adsorption on the carbonized biochar component. The results illustrate that the sorption rate of a solute with biochars is controlled largely by the solutes diffusivity in the biochars partition phase, in which the medium compactness affects directly the solute diffusivity.

Transformation, Morphology, and Dissolution of Silicon and Carbon in Rice Straw-Derived Biochars under Different Pyrolytic Temperatures

Biochars are increasingly recognized as environmentally friendly and cheap remediation agents for soil pollution. The roles of silicon in biochars and interactions between silicon and carbon have been neglected in the literature to date, while the transformation, morphology, and dissolution of silicon in Si-rich biochars remain largely unaddressed. In this study, Si-rich biochars derived from rice straw were prepared under 150-700 °C (named RS150-RS700). The transformation and morphology of carbon and silicon in biochar particles were monitored by FTIR, XRD, and SEM-EDX. With increasing pyrolytic temperature, silicon accumulated, and its speciation changed from amorphous to crystalline matter, while the organic matter evolved from aliphatic to aromatic. For rice straw biomass containing amorphous carbon and amorphous silicon, dehydration (<250 °C) made silicic acid polymerize, resulting in a closer integration of carbon and silicon. At medium pyrolysis temperatures (250-350 °C), an intense cracking of carbon components occurred, and, thus, the silicon located in the inside tissue was exposed. At high pyrolysis temperatures (500-700 °C), the biochar became condensed due to the aromatization of carbon and crystallization of silicon. Correspondingly, the carbon release in water significantly decreased, while the silicon release somewhat decreased and then sharply increased with pyrolytic temperature. Along with SEM-EDX images of biochars before and after water washing, we proposed a structural relationship between carbon and silicon in biochars to explain the mutual protection between carbon and silicon under different pyrolysis temperatures, which contribute to the broader understanding of biochar chemistry and structure. The silicon dissolution kinetics suggests that high Si biochars could serve as a novel slow release source of biologically available Si in low Si agricultural soils.

Quantification of chemical states, dissociation constants and contents of oxygen-containing groups on the surface of biochars produced at different temperatures.

Surface functional groups such as carboxyl play a vital role in the environmental applications of biochar as a soil amendment. However, the quantification of oxygen-containing groups on a biochar surface still lacks systematical investigation. In this paper, we report an integrated method combining chemical and spectroscopic techniques that were established to quantitatively identify the chemical states, dissociation constants (pK(a)), and contents of oxygen-containing groups on dairy manure-derived biochars prepared at 100-700 °C. Unexpectedly, the dissociation pH of carboxyl groups on the biochar surface covered a wide range of pH values (pH 2-11), due to the varied structural microenvironments and chemical states. For low temperature biochars (≤ 350 °C), carboxyl existed not only as hydrogen-bonded carboxyl and unbonded carboxyl groups but also formed esters at the surface of biochars. The esters consumed OH(-) via saponification in the alkaline pH region and enhanced the dissolution of organic matter from biochars. For high temperature biochars (≥ 500 °C), esters came from carboxyl were almost eliminated via carbonization (ester pyrolysis), while lactones were developed. The surface density of carboxyl groups on biochars decreased sharply with the increase of the biochar-producing temperature, but the total contents of the surface carboxyls for different biochars were comparable (with a difference <3-fold) as a result of the expanded surface area at high pyrolytic temperatures. Understanding the wide pKa ranges and the abundant contents of carboxyl groups on biochars is a prerequisite to recognition of the multifunctional applications and biogeochemical cycling of biochars.

H/C atomic ratio as a smart linkage between pyrolytic temperatures, aromatic clusters and sorption properties of biochars derived from diverse precursory materials

Biochar is increasingly gaining attention due to multifunctional roles in soil amelioration, pollution mitigation and carbon sequestration. It is a significant challenge to compare the reported results from world-wide labs regarding the structure and sorption of biochars derived from various precursors under different pyrolytic conditions due to a lack of a simple linkage. By combining the published works on various biochars, we established a quantitative relationship between H/C atomic ratio and pyrolytic temperature (T), aromatic structure, and sorption properties for naphthalene and phenanthrene. A reverse sigmoid shape between T and the H/C ratio was observed, which was independent of the precursors of biochars, including the ash contents. Linear correlations of Freundlich parameters (N, log Kf) and sorption amount (log Qe, log QA) with H/C ratios were found. A rectangle-like model was proposed to predict the aromatic cluster sizes of biochars from their H/C ratios, and then a good structure-sorption relationship was derived. These quantitative relationships indicate that the H/C atomic ratio is a universal linkage to predict pyrolytic temperatures, aromatic cluster sizes, and sorption characteristics. This study would guide the global study of biochars toward being comparable, and then the development of the structure-sorption relationships will benefit the structural design and environmental application of biochars.

Resolution of Adsorption and Partition Components of Organic Compounds on Black Carbons

Black carbons (BCs) may sequester non-ionic organic compounds by adsorption and/or partition to varying extents. Up to now, no experimental method has been developed to accurately resolve the combined adsorption and partition capacity of a compound on a BC. In this study, a unique adsorptive displacement method is introduced to reliably resolve the adsorption and partition components for a solute-BC system. It estimates the solute adsorption on a BC by the use of an adsorptive displacer to displace the adsorbed target solute into the solution phase. The method is validated by tests with uses of activated carbon as the model carbonaceous adsorbent, soil organic matter as the model carbonaceous partition phase, o-xylene and 1,2,3-trichlorobenzene as the reference solutes, and p-nitrophenol as the adsorptive displacer. Thereafter, the adsorption-partition resolution was completed for the two solutes on selected model BCs: four biochars and two National Institute of Standards and Technology (NIST) standard soots (SRM-2975 and SRM-1650b). The adsorption and partition components resolved for selected solutes with given BCs and their dependences upon solute properties enable one to cross-check the sorption data of other solutes on the same BCs. The resolved components also provide a theoretical basis for exploring the potential modes and extents of different solute uptakes by given BCs in natural systems.

Aggregation Kinetics and Self-Assembly Mechanisms of Graphene Quantum Dots in Aqueous Solutions: Cooperative Effects of pH and Electrolytes

The cooperative effects of pH and electrolytes on the aggregation of GQDs and the aggregate morphologies are characterized. Because GQDs have an average size of 9 nm with abundant O-functionalized edges, their suspension was very stable even in a high electrolyte concentration and low pH solution. Divalent cations (Mg2+ and Ca2+) excelled at aggregating the GQD nanoplates, while monovalent cations (Na+ and K+) did not disturb the stability. For Na+ and K+, positive linear correlations were observed between the critical coagulation concentration (CCC) and pH levels. For Mg2+ and Ca2+, negative, but nonlinear, correlations between CCC and pH values could not be explained and predicted by the traditional DLVO theory. Three-step mechanisms are proposed for the first time to elucidate the complex aggregation of GQDs. The first step is the protonation/deprotonation of GQDs under different pH values and the self-assembly of GQDs into GQD-water-GQD. The second step is the self-assembly of small GQD pieces into large plates (graphene oxide-like) induced by the coexisting Ca2+ and then conversion into 3D structures via π-π stacking. The third step is the aggregation of the 3D-assembled GQDs into precipitates via the suppression of the electric double layer. The self-assembly of GQDs prior to aggregation was supported by SEM and HRTEM imaging. Understanding of the colloidal behavior of ultrasmall nanoparticles like GQDs is significantly important for the precise prediction of their environmental fate and risk.

Sorption Behavior of Polycyclic Aromatic Hydrocarbons in Soil–Water System Containing Nonionic Surfactant

This study examined the sorption behavior of PAHs (solutes) in soil-water systems containing surfactants with the objectives of better understanding the fate of contaminants in natural systems and the feasibility of using surfactants for remediation of contaminated soils. Batch studies on the sorption of three PAHs (naphthalene, acenaphthene, and phenanthrene) from water to four different soils with and without a non-ionic surfactant, Triton X-100, were conducted. The ratio of apparent PAH sorption coefficients (Kd*), that is, when Triton X-100 is added, to the intrinsic coefficients (Kd), that is, without the surfactant, varied from greater to less than 1, depending on the system. When Triton X-100 initial concentration (X0) is less than 2CMC (critical micelle concentration), the Kd*/Kd values for all three PAHs are larger than 1 on low-foc soils, which disfavors the use of Triton X-100 for surfactant-enhanced remediation (SER) of contaminated soils. However, for phenanthrene on high-foc soils at low to ...

Surfactant Effects on the Affinity of Plant Cuticles with Organic Pollutants

To precisely predict organics accumulation and crop safety, the affinity of fruit cuticles for naphthalene and 1-naphthol was investigated with the presence of three surfactants below and above the critical micelle concentration (CMC), including anionic sodium dodecylbenzene sulfonate (SDBS), cationic cetyltrimethylammonium bromide (CTMAB), and nonionic polyoxyethylene (20) sorbitan monolaurate (Tween 20). Tomato and apple cuticles with distinct compositions were selected. With increasing SDBS concentrations, apparent sorption coefficients (K(d)*) of 1-naphthol by both cuticles first increased a bit and then decreased slightly. The K(d)* of naphthalene by tomato cuticle is sensitive to SDBS concentration with a sharp increase and then decrease, whereas SDBS has little effect on naphthalene K(d)* by apple cuticle. For CTMAB with lower CMC, the naphthalene K(d)* decreased more quickly. Tween 20 seems to be ineffective on naphthalene sorption by both cuticles. Nevertheless, the intrinsic sorption coefficients (K(d)) were almost promoted by the coexisting surfactants, resulting from the cuticle-sorbed surfactants plasticizing effect.

Dependence of Plant Uptake and Diffusion of Polycyclic Aromatic Hydrocarbons on the Leaf Surface Morphology and Micro-structures of Cuticular Waxes

The uptake of organic chemicals by plants is considered of great significance as it impacts their environmental transport and fate and threatens crop growth and food safety. Herein, the dependence of the uptake, penetration, and distribution of sixteen polycyclic aromatic hydrocarbons (PAHs) on the morphology and micro-structures of cuticular waxes on leaf surfaces was investigated. Plant surface morphologies and wax micro-structures were examined by scanning emission microscopy, and hydrophobicities of plant surfaces were monitored through contact angle measurements. PAHs in the cuticles and inner tissues were distinguished by sequential extraction, and the cuticle was verified to be the dominant reservoir for the accumulation of lipophilic pollutants. The interspecies differences in PAH concentrations cannot be explained by normalizing them to the plant lipid content. PAHs in the inner tissues became concentrated with the increase of tissue lipid content, while a generally negative correlation between the PAH concentration in cuticles and the epicuticular wax content was found. PAHs on the adaxial and abaxial sides of a leaf were differentiated for the first time, and the divergence between these two sides can be ascribed to the variations in surface morphologies. The role of leaf lipids was redefined and differentiated.

Porous PVdF/GO Nanofibrous Membranes for Selective Separation and Recycling of Charged Organic Dyes from Water

Graphene oxide (GO) membranes are robust and continue to attract great attention due to their fascinating properties, despite their potential issues regarding stability and selectivity in aqueous-phase processing. That being said, however, the functional moieties of GO could be used for membrane surface modification, while ensuring simultaneous removal and recycling of industrial organic dyes. Herein, we present a versatile porous structured polyvinylidene fluoride-graphene oxide (PVdF-GO) nanofibrous membranes (NFMs), prepared by using simple and straightforward electrospinning approach for selective separation and filtration. The GO nanosheets were distributed homogeneously throughout the PVdF nanofiber, regulating the surface morphology and performance of PVdF-GO NFM. The PVdF-GO NFMs possesses high mechanical strength and surface free energy (SFE), consequently resulting high permeation and filtration efficiency as compared to PVdF NFM. The selectivity (99%) toward positively charged dyes based on electrostatic attraction, while maintaining rejection (100%) for negatively charged dye from mixed solutions highlight the role of GO in PVdF-GO NFM, owing to uniform pores and negatively charged surface. In addition, the actual efficiency of NFMs could be recovered easily up to three consecutive filtration cycles by regeneration, thereby assuring high stability. The high permeation, purification and filtration efficiency, good stability and recycling of PVdF-GO NFMs are promising for use in practical water purification and applications, particularly for selective filtration and recycling of dyes.